dt-bindings: pinctrl: stm32: Fix missing 'clocks' property in examples

Now that examples are validated against the DT schema, an error with
required 'clocks' property missing is exposed:

Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.example.dt.yaml: \
pinctrl@40020000: gpio@0: 'clocks' is a required property
Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.example.dt.yaml: \
pinctrl@50020000: gpio@1000: 'clocks' is a required property
Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.example.dt.yaml: \
pinctrl@50020000: gpio@2000: 'clocks' is a required property

Add the missing 'clocks' properties to the examples to fix the errors.

Fixes: 2c9239c1 ("dt-bindings: pinctrl: Convert stm32 pinctrl bindings to json-schema")
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>